This invention relates to a combustion device and flue and particularly to an improvement by which the possibility of back draft, that is the reversal of the normal flow of gases in the flue, is prevented or at least reduced.
Various combustion devices are available mounted within buildings in which the heated gases generated in the combustion are extracted from the building by a flue which extends from the combustion device to an outlet in the exterior of the building. In some cases the flue gases are assisted or driven in their movement by a fan arrangement or a flow induction arrangement. This technique is often used in cases where increased heat of combustion is required, in cases where the temperature of the flue gases is reduced by the extraction of heat to a level where they cannot themselves sustain the necessary convection current, or in cases where it is desired that the combustion device operate without a conventional flue stack which is of course necessary to generate the updraft. Examples of these devices are shown in U.S. Pat. Nos. 4,262,608 (Jackson), 4,512,264 (Crawford), 2,979,322 (Dailey), 3,134,345 (King), 2,951,457 (Kneass), 4,149,453 (Reed), 3,570,423 (Hemmingson), 3,527,177 (LaRue), 1,689,241 (Haber), and 4,24,792 (Shimek). In addition similar examples are shown in Canadian Pat. Nos. 487,573 (Campbell), 514,680 (Edwards), and 317,336 (Delamere).
Further patents use a device for assisting the flow through a flue by generating an air stream in the forward direction which induces increased draft. Examples of these devices are shown in U.S. Pat. No. 1,604,271 (Friedman) and 3,175,552 (Sutton). Friedman uses the device to increase combustion when required by a fan which injects air into the combustion chamber and part of that air is bypassed into the flue to increase the draft within the flue. Sutton provides a device which increases the air flow through the flue by an induction system in view of the fact that the normal combustion gas flow is inhibited by heat exchanger positioned in the fireplace.
Other patents relating to steam locomotives show various induction arrangements for improving the combustion. Examples are shown in Canadian Pat. Nos. 239,344, 384,749, 229,808, 216,732, 229,807.
The present invention is concerned with conventional furnaces or woodstoves which generally will when the combustion is operating at a reasonable level generate enough heat in the combustion gases that the flue including a flue stack will cause the updraft to draw the flue gases away from the combustion device.
Conventional chimneys are intended to conduct combustion gases and smoke particles out of the building. However, they can just as well provide a path for movement of air into the building if the air pressure inside is less than the pressure outside. Interior air pressure can become reduced relative to ambient pressure if air is exhausted by bathroom, kitchen and clothes dryer fans and building heating equipment faster than it can be replaced by infiltration of outside air. In such cases, "combustion-venting failure" occurs and smoke is drawn into the building rather than moving along the flue. This can affect health and even cause death due to poisoning by gases such as carbon monoxide or due to fire started when flames or embers are drawn into a room. The problem occurs most often when a fire is first lit or when it is dying because the small fire at that stage does not heat the flue enough or produce sufficient hot gases to overcome pressure differences.
When the flue and the flue gases cool sufficiently, particularly in cold outside temperatures, the convection effect on the vertical column of air in the flue can reverse, causing the movement of air in the flue to reverse in direction due to the fact that it is colder than the interior air of the building. This can cause an inflow of the cold air which may sweep the combustion materials from the combustion device into the building.
This problem is widely experienced particularly where the flue is exterior to the building and hence more susceptible to cooling due to the surrounding cold temperatures.
Despite the wide problem, little effective solution is available at the present time. Attention has been given to the problem of pressure equalization in that in many cases the household now provides an inlet for outside air to ensure that the pressure inside the building does not significantly fall below that of the outside ambient air pressure. However this by itself does not solve the problem since the cooling effect on the flue gases itself can generate the reverse flow or back draft.
The above prior art patents show various devices for inducing air flow in the flue but these generally operate continuously or are operated manually to improve or increase combustion temperatures.